1. Field of the Invention
The present invention relates to vibration type level detectors, and more specifically to a vibration type level detector for detecting a change in the level of a powder and granular material, liquid or the like.
2. Description of the Background Art
In Japanese Patent Laying-Open No.6-102079, a vibration type level detector is described as using a sound piece vibrator. The vibration type level detector includes: a vibration piece of which one end, having its apex portion supported, inserted into a container and having a magnetic substance on a side surface around a leading end of the other end; a vibration generating coil and a reception coil which are arranged to sandwich the magnetic substance on the vibration piece with an appropriate distance therebetween; an amplification portion amplifying a signal from the reception coil and then applying it to the vibration generating coil; and an output portion outputting an output signal in accordance with a reception signal generated in the reception coil.
In the vibration type level detector having the above mentioned structure, when amplification degree in the amplification circuit is increased, the magnetic substance slightly moves at a certain point, and a voltage is induced in the reception coil. Then, the voltage is amplified in the amplification circuit and applied to the vibration generating coil. Thereafter, a magnetic field is generated for suction (repulsion) of the magnetic substance. Thus, the magnetic substance is greatly vibrated, and the voltage generating in the reception coil increases. As a result, vibration of the vibration piece is continued at a constant value. If particles or the like come into contact with the vibration piece inserted into the container while the vibration is continued, the voltage induced in the reception coil decreases as compared with a vibration generating value, and an output signal is output in accordance with the decrease in the voltage.
However, in the vibration type level detector, vibration generated at an electromagnet must efficiently be transferred to the portion of the vibration piece which is in contact with the particles. Therefore, inside portion of a tank is separated from the outside portion, and the vibration piece is supported by a flexible member such as a thin film plate of silicon rubber, for example, so that the vibration of the vibration piece would not be constrained.
In the above described vibration type level detector, as the vibration generating coil and the reception coil are arranged close to each other, the magnetic field generated by the vibration generating coil becomes a large electromotive force when it reaches the reception coil. Thus, it is difficult to detect a slight change in the electromotive force between the magnetic substance and the reception coil due to a slight decrease in the vibrator which depends on the existence or non-existence of the particle, and the particles which are low in relative density cannot be measured.
In addition, the vibration generating coil and the reception coil are arranged to sandwich a diaphragm in a direction which allows the direction of the magnetic field to be orthogonal to the longitudinal direction of the diaphragm, with an appropriate space from the magnetic substance on the side surface of the diaphragm. As a result, a relatively large space is required, so that the compact vibration type level detector cannot be achieved. Further, in mounting the vibration piece to the tank, a sufficient mechanical strength is not obtained as the vibration piece must be supported by the thin film plate so that vibration thereof would not be constrained.
On the other hand, another conventional method is to mount a piezoelectric device which mainly includes titanate lead zirconate (PbTiO.sub.3 -PbZrO.sub.3) to a part of the diaphragm as a vibration generating means, apply a high voltage from an amplification circuit to the piezoelectric device, transfer mechanical distortion generated from the piezoelectric device to the diaphragm, and generate vibration necessary for the measurement.
However, a driving voltage applied to the piezoelectric device and a intensity of the generated vibration (amplitude by vibration) are in a direct proportion. Thus, the voltage is high as compared with a usual circuit voltage and is generally between several tens of volts to several hundreds of volts. Thus, another circuit for internally generating a voltage is required.
Further, a breakdown voltage or insulation performance of the portion to which the high voltage is applied must carefully be considered, and there is a danger of an electric shock if a person inattentively touches it. Besides the fact that the applied voltage is high, the piezoelectric device itself generates a high voltage by impact or vibration. As a result, atmospheric discharge may be caused by the high voltage, and the use of the detector in inflammable gas or inflammable vapor is highly restricted.
In addition, a ferroelectric substance such as titanate lead zirconate, which is used as a material for the piezoelectric device, has a crystalline structure called a perovskite form, which has a pyramidal quadratic system at a room temperature, and a piezoelectric effect is obtained by spontaneous polarization. However, it loses the spontaneous polarization as the structure thereof changes to a cubic system due to increase in temperature, whereby the piezoelectric effect can no longer be obtained. The temperature at which the phase of the crystalline structure changes from the pyramidal quadratic system to the cubic system is called a Curie point. The piezoelectric effect gradually decreases as the temperature rises from the room temperature toward the Curie point, at which point no piezoelectric effect is obtained. Thus, when the piezoelectric device is used as a vibration generating means, it is impossible to use it at the temperature above the Curie point, and even below the Curie point, any change in the temperature results in a change in a vibration generating efficiency and, strictly speaking, results in a variation in detection sensitivity.